Fix git features not working when a Windows host collaborates with a unix guest (#43515)

We were using `std::path::Path::strip_prefix` to determine which
repository an absolute path belongs to, which doesn't work when the
paths are Windows-style but the code is running on unix. Replace it with
a platform-agnostic implementation of `strip_prefix`.

Release Notes:

- Fixed git features not working when a Windows host collaborates with a
unix guest

crates/git_ui/src/git_panel.rs

@@ -4351,8 +4351,11 @@ impl GitPanel {
                             .when(strikethrough, Label::strikethrough),
                     ),
                     (true, false) => this.child(
-                        self.entry_label(format!("{dir}{}", path_style.separator()), path_color)
-                            .when(strikethrough, Label::strikethrough),
+                        self.entry_label(
+                            format!("{dir}{}", path_style.primary_separator()),
+                            path_color,
+                        )
+                        .when(strikethrough, Label::strikethrough),
                     ),
                     _ => this,
                 }